1. Field of the Invention
This invention relates to a color video signal recording and/or reproducing apparatus and, more particularly, to apparatus for providing precise color frame lock of color television signals, even if magnetic tape on which the color television signals, like the NTSC signal, has been recorded is spliced electrically or physically.
2 Description of the Prior Art
As is well known, in the NTSC color television signal, there is an exact frequency relationship between the horizontal synchronizing frequency f.sub.h and the color subcarrier frequency f.sub.sc, namely f.sub.sc = 455/2f.sub.h, and consequently four television fields must occur before the color subcarrier signal exactly repeats itself in phase with respect to the horizontal synchronizing signal. In other words, the periodicity of the color frame is four fields. Assuming that the color subcarrier signal has its positive peak value at the front edge of the horizontal synchronizing signal, the color subcarrier signal has its negative peak value at the front edge of the next horizontal synchronizing signal. This means that the phase of the subcarrier signal is reversed at every horizontal interval. As a result of the reversal, if the subcarrier has its negative peak value at the front edge of the first equalizing pulse included in the first field of a first frame, the positive peak value of the subcarrier signal occurs at the front edge of the first equalizing pulse included in the first field of the next frame, which immediately succeeds the first frame. In that sense, these two frames are different. It will be evident that if a continuous signal is to be reproduced, splices must join successive frames in the correct sequence; i.e., the first frame must be joined to the second frame. If the first frame is joined to another frame having the same phase relationship between the color subcarrier and the first equalizing pulse as the first frame so as to constitute another "first" frame, there will be a sudden 180.degree. phase shift in the burst or color subcarrier signals at the splicing point.
In the conventional color television receiver, the color subcarrier signal used for synchronous detection is formed on the basis of the burst signals. The subcarrier signal forming circuit has a fly-wheel effect to some extent, so that even if the phase of the burst signal is suddenly reversed, the phase of the subcarrier signal cannot follow the sudden change of the phase of the burst signal. As a result, there will be some phase differences between the chrominance signal and the color subcarrier signal, and hence there will be a transient disturbance of the hue of a reproduced picture.
This is an obviously unacceptable condition, and the normal VTR is provided with means for recognizing the improper phase and shifting the phase of the whole television signal by half a cycle of the subcarrier signal to bring it back into the proper phase. In order to perform the above operation, the VTR has a delay line, to which the color television signal is applied. The burst signal is separated from the composite signal and then the phase of the burst signal is compared in a comparator with the phase of a reference subcarrier signal. If the phase of the color subcarrier signal is reversed at the editing point, an error voltage will be obtained from the comparator. The error voltage is supplied to the delay line to shift the timing of the whole signal, and this the latter will be moved 180.degree. (140 nano seconds) ahead or behind the proper timing position. In other words, the phase correction places the color subcarrier signal in proper phase, but it introduces a 140 nano second error in the horizontal timing. Thus, the insertion or removal of 140 nsec. of delay at the editing point causes the picture on the screen to jump sideways.
In order to avoid the above-described disadvantage, several approaches and methods have been proposed. One method is to use the 15Hz frame pulses instead of the 30Hz pulses used on the control track. This refers to the use of frame pulses at one-fourth the basic field repetition rate of NTSC signals. This means that a servo operation is performed once every four fields. Accordingly, the lock-up time of the servo operation will be increased by approximately 20 percent in comparison with the 30Hz servo operation. Also, it is difficult to splice the tape at the exact field or frame.
Another method is to use a servo control circuit in which, if the error voltage from the burst phase comparator indicates lock-up to the wrong frame, an electrical signal is generated which causes the tape drive motor momentarily to speed up so as to move the magnetic tape ahead by a distance corresponding to approximately one frame. However, in this method, the servo circuit must be unlocked once the error voltage is detected, and thereafter the servo circuit must be operated to lock in the new frame again. This means that the total lock-up time of the VTR is greatly increased, as in the above-described method.